Vaporizer

ABSTRACT

A vaporizer for delivery of a volatile medium to a gas flow, the vaporizer comprising: a gas delivery unit ( 3 ) which receives a flow of gas and provides a flow of gas containing a metered amount of a vaporized medium; a reservoir unit ( 5 ) which contains a volatile medium and maintains a supply of the vaporized medium, wherein the reservoir unit is selectively fluidly connected to the gas delivery unit; a gas sensing unit ( 7 ) for sensing a flow rate and/or composition of the gas flow; a vaporized medium sensing unit ( 9 ) for sensing a flow rate of the vaporized medium; a manifold ( 10 ) which includes flow paths for the vaporized medium and fluidly connects the reservoir unit and the vaporized medium sensing unit; and a control unit ( 11 ) for controlling a flow rate of the gas flow and an amount of the vaporized medium which is metered into the gas flow.

The present invention relates to a vaporizer, especially for thedelivery of a volatile anaesthetic to a fresh gas flow, and componentsof such a vaporizer.

Volatile anaesthetics, such as desflurane, are metered into a fresh gasflow which is delivered to a patient for the purpose of achievinggeneral anaesthesia. Other volatile anaesthetics include, for example,halothane, enflurane, isoflurane and sevoflurane.

In one aspect the present invention provides a vaporizer for delivery ofa volatile medium to a gas flow, the vaporizer comprising: a gasdelivery unit which receives a flow of gas and provides a flow of gascontaining a metered amount of a vaporized medium; a reservoir unitwhich contains a volatile medium and maintains a supply of the vaporizedmedium, wherein the reservoir unit is selectively fluidly connected tothe gas delivery unit; a gas sensing unit for sensing a flow rate and/orcomposition of the gas flow; a vaporized medium sensing unit for sensinga flow rate of the vaporized medium; a manifold which includes flowpaths for the vaporized medium and fluidly connects the reservoir unitand the vaporized medium sensing unit; and a control unit forcontrolling a flow rate of the gas flow and an amount of the vaporizedmedium which is metered into the gas flow.

In another aspect the present invention provides a vaporizer fordelivery of a volatile medium to a gas flow, the vaporizer comprising: agas delivery unit which receives a flow of gas and provides a flow ofgas containing a metered amount of a vaporized medium; a reservoir unitwhich contains a volatile medium and maintains a supply of the vaporizedmedium, wherein the reservoir unit is selectively fluidly connected tothe gas delivery unit; and a control unit for controlling a flow rate ofthe gas flow and an amount of the vaporized medium which is metered intothe gas flow; wherein the reservoir unit comprises a tank which containsthe volatile medium in liquid form, and a heater which is operative toheat the volatile medium to a vaporization temperature in order tovaporize the volatile medium and maintain an operating pressure in aheadspace of the tank, the tank being formed of a thermally-conductivematerial and providing a thermal mass which is sufficient to maintainthe contained volatile medium at the vaporization temperature for atleast 3 minutes, optionally for at least 5 minutes, followingde-actuation of the heater.

In a further aspect the present invention provides a vaporizer fordelivery of a volatile medium to a gas flow, the vaporizer comprising: agas delivery unit which receives a flow of gas and provides a flow ofgas containing a metered amount of a vaporized medium; a reservoir unitwhich contains a volatile medium and maintains a supply of the vaporizedmedium, wherein the reservoir unit is selectively fluidly connected tothe gas delivery unit; and a control unit for controlling a flow rate ofthe gas flow and an amount of the vaporized medium which is metered intothe gas flow; wherein the reservoir unit comprises a filler assembly bywhich volatile medium is introduced into the tank, the filler assemblycomprising a body which includes a port for receiving a fillingcontainer, a chamber which is fluidly connected to the tank, and a valveassembly which, when opened, is operative to open a fluid connectionbetween the port and the chamber so as to provide for filling of thetank with volatile medium from the filling container; wherein the fillerassembly comprises a loading/unloading mechanism which is operative toengage the filling container and load/unload the filling container tothe valve assembly, the loading/unloading mechanism comprising anengagement member which is adapted to engage the body of the fillingcontainer when the filling container is inserted into the port of thebody of the filler assembly, and a lever assembly which is coupled tothe engagement member and operative between a first, unlocked positionand a second, locked position in which the valve assembly is open toallow for filling of the tank by the filling container.

In a still further aspect the present invention provides a vaporizer fordelivery of a volatile medium to a gas flow, the vaporizer comprising: agas delivery unit which receives a flow of gas and provides a flow ofgas containing a metered amount of a vaporized medium; a reservoir unitwhich contains a volatile medium and maintains a supply of the vaporizedmedium, wherein the reservoir unit is selectively fluidly connected tothe gas delivery unit; a gas sensing unit for sensing a flow rate and/orcomposition of the gas flow; and a control unit for controlling a flowrate of the gas flow and an amount of the vaporized medium which ismetered into the gas flow; wherein the gas sensing unit comprises a gasinlet port through which the flow of gas is delivered, an outlet portfrom which the flow of gas is delivered, a flow path which fluidlyconnects the gas inlet port and the outlet port, and at least one pairof low rate sensors for sensing the flow rate of the gas flow; whereinthe flow rate sensors of each pair of flow rate sensors are physicallyseparated along a length of the flow path, providing an upstream sensorand a downstream sensor, and the upstream sensor includes a temperaturesensor element and the downstream sensor includes a heater element and atemperature sensor element, the upstream sensor being configured tosense the ambient temperature of the gas flow and the downstream sensorbeing configured to determine an energy required to maintain the heaterelement at a required temperature differential in relation to theambient temperature as determined by the upstream sensor, whereby theflow rate of the gas flow is determined from an energy required tomaintain the heater element at the required temperature differential.

In a yet further aspect the present invention provides a vaporizer fordelivery of a volatile medium to a gas flow, the vaporizer comprising: agas delivery unit which receives a flow of gas and provides a flow ofgas containing a metered amount of a vaporized medium; a reservoir unitwhich contains a volatile medium and maintains a supply of the vaporizedmedium, wherein the reservoir unit is selectively fluidly connected tothe gas delivery unit; a gas sensing unit for sensing a flow rate and/orcomposition of the gas flow; and a control unit for controlling a flowrate of the gas flow and an amount of the vaporized medium which ismetered into the gas flow; wherein the gas sensing unit comprises a gasinlet port through which the flow of gas is delivered, an outlet portfrom which the flow of gas is delivered, a flow path which fluidlyconnects the gas inlet port and the outlet port, and at least one flowrate sensor for sensing the flow rate of the gas flow; wherein the flowpath includes a stub path, and the gas sensing unit further comprises atleast one gas characteristic sensor in the stub path for measuring acharacteristic of the gas flow which is representative of a compositionof the gas flow, with the measurement from the at least one gascharacteristic sensor being used to provide a compensation factor forthe flow rate of the gas flow as measured by the at least one flow ratesensor.

In a still yet further aspect the present invention provides a vaporizerfor delivery of a volatile medium to a gas flow, the vaporizercomprising: a gas delivery unit which receives a flow of gas andprovides a flow of gas containing a metered amount of a vaporizedmedium; a reservoir unit which contains a volatile medium and maintainsa supply of the vaporized medium, wherein the reservoir unit isselectively fluidly connected to the gas delivery unit; a gas sensingunit for sensing a flow rate and/or composition of the gas flow; avaporized medium sensing unit for sensing a flow rate of the vaporizedmedium; and a control unit for controlling a flow rate of the gas flowand an amount of the vaporized medium which is metered into the gasflow; wherein the vaporized medium sensing unit comprises a body whichis thermally connected to the reservoir unit and includes a flow channelwhich includes an inlet port which is fluidly connected to the reservoirunit and an outlet port which is fluidly connected to the gas deliveryunit, and at least one flow sensor for detecting a flow rate of thevaporized medium through the flow channel.

Preferred embodiments of the present invention will now be describedhereinbelow by way of example only with reference to the accompanyingdrawings, in which:

FIG. 1 illustrates a front perspective view of a vaporizer in accordancewith one embodiment of the present invention;

FIG. 2 illustrates a rear perspective view of the vaporizer of FIG. 1;

FIG. 3(a) illustrates a front perspective view of the reservoir unit ofthe vaporizer of FIG. 1;

FIG. 3(b) illustrates a rear perspective view of the reservoir unit ofFIG. 3(a);

FIG. 3(c) illustrates a part cut-away perspective view of the reservoirunit of FIG. 3(a);

FIG. 4(a) illustrates a first front perspective view of the fillerassembly of the reservoir unit of FIG. 3(a);

FIG. 4(b) illustrates a second front perspective view of the fillerassembly of FIG. 4(a);

FIG. 4(c) illustrates a sectional view (along section I-I in FIG. 4(a))of the filler assembly of FIG. 4(a);

FIG. 4(d) illustrates a sectional view (along section I-I in FIG. 4(a))of the filler assembly of FIG. 4(a), with a filling container fittedthereto in a first, rest position;

FIG. 4(e) illustrates a sectional view (along section I-I in FIG. 4(a))of the filler assembly of FIG. 4(a), with a filling container fittedthereto in a second, filling position;

FIGS. 5(a) and (b) illustrate perspective views of a modified fillerassembly for the vaporizer of FIG. 1;

FIG. 6(a) illustrates a perspective view of the fresh gas sensing unitof the vaporizer of FIG. 1;

FIG. 6(b) illustrates a part cut-away fragmentary perspective view ofthe fresh gas sensing unit of FIG. 6(a);

FIG. 7 illustrates a part cut-away fragmentary perspective view of amodified fresh gas sensing unit for the vaporizer of FIG. 1;

FIG. 8(a) illustrates a perspective view of the vaporized medium flowsensor of the vaporizer of FIG. 1;

FIG. 8(b) illustrates an exploded perspective view of the vaporizedmedium flow sensor of FIG. 8(a);

FIG. 8(c) illustrates a first vertical sectional view (along sectionII-II in FIG. 8(a)) through the vaporized medium flow sensor of FIG.8(a); and

FIG. 8(d) illustrates a second vertical sectional view (along sectionIII-III in FIG. 8(a)) through the vaporized medium flow sensor of FIG.8(a).

The vaporizer comprises a fresh gas delivery unit 3 which receives aflow of fresh gas and provides a flow of fresh gas containing a meteredamount of a vaporized medium, a reservoir unit 5 which contains avolatile medium and maintains a supply of the vaporized medium, a freshgas sensing unit 7 for sensing a flow rate and composition of the freshgas, a vaporized medium sensing unit 9 for sensing a flow rate of thevaporized medium, a manifold 10 which includes flow paths for thevaporized medium, and a control unit 11 for controlling the flow rate ofthe fresh gas flow and the amount of the vaporized medium which ismetered into the fresh gas flow.

The fresh gas delivery unit 3 comprises an inlet 15 which is connectedto a supply of fresh gas and an outlet 17 which provides a flow of freshgas containing a metered amount of a vaporized medium.

In this embodiment the fresh gas comprises a mixture of oxygen, air andnitrous oxide, but can have any desired form.

As particularly illustrated in FIGS. 3(a) to (c), the reservoir unit 5comprises a tank 21 which contains the volatile medium in liquid form, aheater 23 which is operative to heat the volatile medium to avaporization temperature in order to vaporize the volatile medium andmaintain an operating pressure in a headspace of the tank 21, a fillerassembly 25 by which volatile medium is introduced into the tank 21, anda valve assembly 27 by which the vaporized medium which is delivered tothe fresh gas flow can be metered and shut off.

In this embodiment the tank 21 is formed of a thermally-conductivematerial and, together with the manifold 10, is of sufficient thermalcapacity in order maintain the contained volatile medium at thevaporization temperature for a predetermined period of time, in thisembodiment at least 3 minutes and preferably at least 5 minutes,following de-actuation of the heater 23. With this configuration, byproviding the tank 21 with a thermal store, the vaporizer can continueto vaporize the volatile medium without provision of power to the heater23, either from mains or battery, and optionally is configured to enablea vaporized medium delivery rate of at least 1.2 L/min for thepredetermined period of time. In existing systems, power is typicallymaintained to the heater 23 by use of a battery.

In this embodiment the tank 21 is defined in part by the manifold 10.

In this embodiment the tank 21 is formed of aluminum, which ishighly-thermally conductive. In preferred embodiments the tank 21 couldbe formed of any material which is compatible with the volatile mediumand has a thermal conductivity of at least about 100 W/(m.K), optionallyat least about 150 W/(m.K), optionally at least about 200 W/(m.K). In analternative embodiment the tank 21 or at least parts thereof could beformed of brass.

In this embodiment, as particularly illustrated in FIGS. 4(a) to (e),the filler assembly 25 comprises a body 31 which includes a port 33 forreceiving a filling container F, here a bottle, a chamber 35 which isfluidly connected to the tank 21, and a valve assembly 37 which, whenopened, is operative to open a fluid connection between the port 33 andthe chamber 35 so as to provide for filling of the tank 21 with volatilemedium from the filling container F.

In this embodiment the valve assembly 37 comprises a bore 41 whichsealingly receives an external nozzle N of the filling container F andincludes at least one valve opening 42 at one, distal end thereof, aseat 43 which receives a valve element V of the filling container F,which is internal to the nozzle N and when depressed opens a fluidconnection between the nozzle N and a body of the filling container F,and a valve element 47 which is normally biased to close the at leastone valve opening 42 and is engaged by the nozzle N of the fillingcontainer F to displace the valve element 47 and open the at least onevalve opening 42.

In this embodiment the valve element 47 includes at least one, here aplurality of projections 49 which project through the at least one valveopening 42, and are engaged by the nozzle N of the filling container Fto displace the valve element 47.

In this embodiment the seat 43 extends proximally of the projections 49of the valve element 47. With this configuration, inadvertent operationof the valve element 47 by introduction of, for example, a finger intothe bore 41, is prevented.

The filler assembly 25 further comprises a loading/unloading mechanism51 which is operative to engage the filling container F and load/unloadthe filling container F to the valve assembly 37.

In this embodiment the loading/unloading mechanism 51 comprises anengagement member 53 which is adapted to engage a body of the fillingcontainer F, in this embodiment the neck K of the filling container F,when the filling container F is inserted into the port 33 of the body 31of the filler assembly 25, and a lever assembly 55 which is coupled tothe engagement member 53 and is operative between a first, unlockedposition (as illustrated in FIG. 4(d)) and a second, locked position (asillustrated in FIG. 4(e)) in which the valve assembly 37 is open toallow for filling of the tank 21 by the filling container F.

In this embodiment the lever assembly 55 comprises a lever 57, herecomprising first and second arms, which is pivotally coupled to the body31 of the filler assembly 25, and a coupling 59, here comprising firstand second rods, which couples the lever 57 to the engagement member 53.

In operation, the nozzle N of the filling container F is inserted intothe bore 41 of the valve assembly 37, with the lever assembly 55 in thefirst, unlocked position, and, on moving the lever 57 to the second,locked position, the engagement member 53 is moved in relation to thebody 31 of the filler assembly 25, which causes the valve element V ofthe filling container F to engage the seat 43 of the valve assembly 37and the nozzle N of the filling container F to engage the valve element47 of the valve assembly 37, which causes the valve element V of thefilling container F and the valve element 47 of the valve assembly 37 tobe opened. In this embodiment the nozzle N of the filling container Fengages the valve element 47 of the valve assembly 37 ahead of the valveelement V of the filling container F engaging the seat 43 of the valveassembly 37, such that the valve element 47 of the filler assembly 25 isopened ahead of the valve element V of the filling container F.

In this embodiment the engagement member 53 includes a slide element 61which is movable laterally and acts to engage the body of the fillingcontainer F, here the neck K of the filling container F, such as to fixthe filling container F to the engagement member 53.

In this embodiment the slide element 61 includes a recess 62, which,when the engagement member 53 is moved to the locked position, islocated at a detent 63, here a projection, in order to prevent slidingof the slide element 61 and inadvertent release of the filling containerF from the engagement member 53 during a filling operation. In analternative embodiment the recess 62 could instead be a projection.

In one embodiment the recess 62 and the detent 63 can be configured suchthat the slide element 61 is moved to an open position when the leverassembly 55 is moved to the unlocked position.

In an alternative embodiment the lever assembly 55 could be configuredsuch as to be movable from the unlocked position to a release positionby movement of the lever 57 is a sense opposite to that in which thelever 57 is moved to the locked position, and the recess 62 and thedetent 63 can be configured such that the slide element 61 is moved toan open position when the lever assembly 55 is in the release position.

In this embodiment the filler assembly 25 is thermally connected to thetank 21.

FIGS. 5(a) and (b) illustrate a modification to the filler assembly 25of FIGS. 4(a) to (e).

In this embodiment the slide element 61 comprises at least one slidepart 64 which is slideably rotated about at least one pivot 65, herecomprising first and second slide parts 64 a, b which are rotated aboutfirst and second pivots 65 a, b, and the loading/unloading mechanism 51further comprises a guide member 66 which acts to cause the slide parts64 a, b to slide laterally when the loading/unloading mechanism 51 ismoved axially between the unlocked (as illustrated in FIG. 5(a)) andlocked (as illustrated in FIG. 5(b)) positions.

In this embodiment each slide part 64 a, b includes a projection 67,here at a distal end to the pivot 65 a, b, and the guide member 66includes at least one guide 68, here first and second guides 68 a, b,which receive the projections 67 of the respective slide parts 64 a, b.

In this embodiment the guides 68 a, b include a first guide section 69,in which the projections 67 are located when the loading/unloadingmechanism 51 is in the unlocked position, and a second guide section 70which is disposed laterally inwardly of the first guide section 69, inwhich the projections 67 are located when the loading/unloadingmechanism 51 is in the locked position.

With this configuration, when the loading/unloading mechanism 51 is inthe unlocked position, the projections 67 of the slide parts 64 a, b arelocated in the first guide sections 69 of the guides 68 a, b and theslide parts 64 a, b have a first, laterally-outward position whichallows the neck K of the filling container F to be inserted into thebore 41 of the valve assembly 37, and, when the loading/unloadingmechanism 51 is moved to the locked position, in this embodiment byaxial displacement of the engagement member 53, the projections 67 ofthe slide parts 64 a, b are located in the second guide sections 70 ofthe guides 68 a, b and the slide parts 64 a, b have a second,laterally-inward position in which the neck K of the filling container Fis engaged by the slide parts 64 a, b and prevents release of thefilling container F during a filling operation. When theloading/unloading mechanism 51 is returned to the unlocked position, theprojections 67 of the slide parts 64 a, b are returned to the firstguide sections 69 of the guides 68 a, b and the slide parts 64 a, b havethe first, laterally-outward position which allows the neck K of thefilling container F to be removed from the bore 41 of the valve assembly37.

In this embodiment the guides 68 a, b each have the form of a track,here formed by a through slot.

In this embodiment the valve assembly 27 comprises a shut-off valve 71which is operated as required to prevent delivery of the vaporizedmedium into the fresh gas flow, and a metering valve 72 which regulatesthe amount of the vaporized medium which is metered into the fresh gasflow in accordance the measured flow rate of the fresh gas flow, as willbe described in more detail hereinbelow.

In this embodiment the shut-off valve 71 includes an inlet port 73 whichis fluidly connected to the tank 21 of the reservoir unit 5 through aflow path in the manifold 10, and an outlet port 74 which is fluidlyconnected to an inlet port 75 of the metering valve 72 through a flowpath in the manifold 10.

In this embodiment the metering valve 72 includes an inlet port 75 whichis fluidly connected to the outlet port 74 of the shut-off valve 72through a flow path in the manifold 10, and an outlet port 76 which isfluidly connected to the vaporized medium sensing unit 9 through a flowpath in the manifold 10.

In this embodiment, as particularly illustrated in FIGS. 6(a) and (b),the fresh gas sensing unit 7 comprises a fresh gas inlet port 77 throughwhich the flow of fresh gas is delivered, an outlet port 78 from whichthe flow of fresh gas is delivered, a flow path 79 which fluidlyconnects the fresh gas inlet port 77 and the outlet port 78, and atleast one flow rate sensor 80 for sensing the flow rate of the fresh gasflow.

In this embodiment the flow path 79 has a diameter of 5 mm.

In this embodiment the fresh gas sensing unit 7 comprises at least onepair of flow rate sensors 80, here first and second pairs of flow ratesensors 80 a, 80 b.

In this embodiment the flow rate sensors 80 a, 80 b are mass flow ratesensors, here mass flow sensors, which comprise a heater element and atemperature sensor element.

By providing a pair of flow rate sensors 80, each physically separatedalong a length of the flow path, the fresh gas sensing unit 7 canprovide a more accurate measurement. The present inventors havedetermined that a more accurate measurement can be obtained byconfiguring the pair of flow rate sensors 80 such as to utilize theheater and temperature sensor elements of the downstream sensor 80 andonly the temperature sensor element of the upstream sensor 80 inperforming a flow rate measurement.

In this embodiment the upstream sensor 80 is configured to sense theambient temperature of the fresh gas flow and the downstream sensor 80is configured to determine the energy required to maintain the heaterelement at a required temperature differential in relation to theambient temperature as determined by the upstream sensor 80, whichenergy determines the flow rate of the fresh gas flow. In thisembodiment the downstream sensor 80 heats the fresh gas flow to apredetermined temperature differential in relation to, here 30° C.above, the ambient temperature as determined by the upstream sensor 80.

Furthermore, by providing first and second pairs of flow rate sensors 80a, 80 b, a control check is provided, in that the control system canmonitor the output of each of the pairs of flow rate sensors 80 a, 80 b,and, if the measured output exceeds a predetermined threshold, raise analert and/or shut down the vaporizer.

In this embodiment the flow path 79 includes a stub path 81, and thefresh gas sensing unit 7 further comprises at least one fresh gascharacteristic sensor 83 in the stub path 81.

In this embodiment the stub path 81 is downstream of the at least oneflow rate sensor 80 a, 80 b.

In an alternative embodiment the stub path 81 could be upstream of theat least one flow rate sensor 80 a, 80 b.

In this embodiment the stub path 81 is located in a substantially linearsection of the flow path 79.

In this embodiment the stub path 81 comprises a sensor cavity 85 atwhich the at least one fresh gas characteristic sensor 83 is located,and a flow restriction 87 which fluidly connects the flow path 79 to thesensor cavity 85.

In this embodiment the at least one fresh gas characteristic sensor 83is a mass flow rate sensor, here a mass flow sensor. In this embodimentthe at least one fresh gas characteristic sensor 83 is of the same kindas the flow rate sensors 80 a, 80 b.

In this embodiment the flow restriction 87 comprises a flow channel ofsmaller diameter than the flow path 79, here of a diameter of 1.75 mm.In preferred embodiments the flow restriction has an area of about 1.5mm² to about 3.5 mm², preferably about 2 mm² to about 3 mm².

With this configuration, the flow restriction 87 provides for anextended residence time of the fresh gas in the sensor cavity 85, whichin effect provides that the at least one fresh gas characteristic sensor83 provides for a “zero flow” or “static” measurement of acharacteristic of the gas flow which is representative of composition.In this embodiment the average residence time in the sensor cavity 85 isat least 20 seconds, optionally at least 25 seconds, optionally at least30 seconds, and optionally at most 60 seconds. In this embodiment theaverage residence time in the sensor cavity 85 is at most 60 seconds,optionally at most 40 seconds, and optionally at most 35 seconds. Thepresent inventors have established that this “zero flow” or “static”measurement of the fresh gas flow enables a more precise determinationof the flow rate of the fresh gas flow when the measurement from the atleast one fresh gas characteristic sensor 83 is used to provide acompensation factor for the flow rate of the fresh gas flow as measuredby the flow rate sensors 80 a, 80 b.

In this embodiment the measurement from the at least one fresh gascharacteristic sensor 83 provides a compensation factor, and, for thiscompensation factor, a look-up table is selected, from which acompensated flow rate is determined.

In an alternative embodiment the compensation factor can be applied to afitted function for a flow rate as measured by the flow rate sensors 80a, 80 b.

In a further embodiment the measurement from the at least one fresh gascharacteristic sensor 83 provides a compensation factor, and, for thiscompensation factor, a look-up table is selected, from which acompensated flow rate is determined and applied to a fitted function fora flow rate as measured by the flow rate sensors 80 a, 80 b.

In this embodiment the fresh gas sensing unit 7 includes a vaporizedmedium inlet port 89 in the flow path 79 thereof and downstream of theat least one fresh gas characteristic sensor 83, which is fluidlyconnected to the outlet port 97 of the vaporized medium sensing unit 9,by which the vaporized medium is metered into the flow of fresh gas.

In one alternative embodiment, as illustrated in FIG. 7, the flowrestriction 87 could comprise a porous element 91, here a sinteredelement, which is located in the stub path 81.

In this embodiment, as particularly illustrated in FIGS. 8(a) to (d),the vaporized medium sensing unit 9 comprises a body 92 which includes aflow channel 93 which includes an inlet port 95 which is fluidlyconnected to the outlet port 76 of the metering valve 72 and an outletport 97 through which a flow of the vaporized medium is delivered, andat least one flow sensor 101 for detecting a flow rate of the vaporizedmedium through the flow channel 93.

In this embodiment the body 92 comprises first and second body parts 103a, b and an annular seal 105 which surrounds the flow channel 93.

In this embodiment at least one of the first and second body parts 103a, b of the body 92 is formed of a material of high thermalconductivity.

In one embodiment at least one of the first and second body parts 103 a,b of the body 92 is formed of the same material as the tank 21 of thereservoir unit 5.

In this embodiment the at least one of the first and second body parts103 a, b of the body 92 is thermally connected, here fixed directly, tothe tank 21 of the reservoir unit 5. With this configuration, the body92 is maintained in a heated state, which prevents condensation in theflow channel 93, which could hinder accurate detection of the flow rateof the vaporized medium.

In this embodiment the flow channel 93 is a linear channel.

In this embodiment the flow channel 93 has a cross-sectional area of 2mm², and in preferred embodiments has a cross-sectional area from about1.5 mm² to about 2.5 mm². This configuration has been found to providefor establishment of a laminar flow over the at least one flow sensor101 for a broad range of flows.

In this embodiment the at least one flow sensor 101 is a mass flow ratesensor, here a mass flow sensor.

In this embodiment the vaporized medium sensing unit 9 includes firstand second flow sensors 101 a, b. By providing first and second flowsensors 101 a, b, a control check is provided, in that the controlsystem can monitor the output of each of the flow sensors 101 a, b, and,if the measured output exceeds a predetermined threshold, raise an alertand/or shut down the vaporizer.

In this embodiment the manifold 10 comprises a thermal block 111 of amaterial of high thermal conductivity which includes flow passages forthe vaporized medium. By being thermally connected to the tank 21 of thereservoir unit 5, which is heated by the heater 23, the manifold 10 ismaintained in a heated state, which prevents condensation of thevaporized medium in the flow passages.

In one embodiment the thermal block 111 is formed of the same materialas the tank 21 of the reservoir unit 5.

In this embodiment the manifold 10 includes a connector 115 whichfluidly connects the outlet port 97 of the vaporized medium sensing unit9, here through a flow passage in the thermal block 111, to vaporizedmedium inlet port 89 in the flow path 79 of the fresh gas sensing unit7.

In one embodiment the connector 115 is formed of the same material asthe tank 21 of the reservoir unit 5.

Finally, it will be understood that the present invention has beendescribed in its preferred embodiments and can be modified in manydifferent ways without departing from the scope of the invention asdefined in the appended claims.

For example, although the present invention has been described withreference to the delivery of a general anaesthetic, it will beunderstood that the present invention has application to thevaporization of any medium for delivery in a gas flow.

1. A vaporizer for delivery of a volatile medium to a gas flow, thevaporizer comprising: a gas delivery unit which receives a flow of gasand provides a flow of gas containing a metered amount of a vaporizedmedium; a reservoir unit which contains a volatile medium and maintainsa supply of the vaporized medium, wherein the reservoir unit isselectively fluidly connected to the gas delivery unit; a gas sensingunit for sensing a flow rate and/or composition of the gas flow; avaporized medium sensing unit for sensing a flow rate of the vaporizedmedium; a manifold which includes flow paths for the vaporized mediumand fluidly connects the reservoir unit and the vaporized medium sensingunit; and a control unit for controlling a flow rate of the gas flow andan amount of the vaporized medium which is metered into the gas flow. 2.The vaporizer of claim 1, wherein the gas delivery unit comprises aninlet which is connected to a supply of gas and an outlet which providesa flow of gas containing a metered amount of the vaporized medium. 3.The vaporizer of claim 1 or 2, wherein the reservoir unit comprises atank which contains the volatile medium in liquid form, and a heaterwhich is operative to heat the volatile medium to a vaporizationtemperature in order to vaporize the volatile medium and maintain anoperating pressure and vaporized medium in a headspace of the tank. 4.The vaporizer of claim 3, wherein the reservoir unit further comprises avalve assembly by which the vaporized medium which is delivered from thetank to the gas flow can be metered.
 5. The vaporizer of claim 3 or 4,wherein the tank is formed of a thermally-conductive material and,optionally together with the manifold, provides a thermal mass which issufficient to maintain the contained volatile medium at the vaporizationtemperature for at least 3 minutes, optionally for at least 5 minutes,following de-actuation of the heater.
 6. The vaporizer of claim 5,wherein the tank is configured to maintain the contained volatile mediumat the vaporization temperature while delivering the vaporized medium ata required flow rate, optionally at a flow rate of at least 1 L/min. 7.The vaporizer of any of claims 3 to 6, wherein the tank is defined inpart by the manifold.
 8. The vaporizer of any of claims 3 to 7, whereinthe tank is formed of a material which has a thermal conductivity of atleast about 100 W/(m.K), optionally at least about 150 W/(m.K),optionally at least about 200 W/(m.K), optionally of aluminum or analuminum alloy.
 9. The vaporizer of any of claims 3 to 8, wherein thereservoir unit further comprises a filler assembly by which volatilemedium is introduced into the tank.
 10. The vaporizer of claim 9,wherein the filler assembly comprises a body which includes a port forreceiving a filling container, a chamber which is fluidly connected tothe tank, and a valve assembly which, when opened, is operative to opena fluid connection between the port and the chamber so as to provide forfilling of the tank with volatile medium from the filling container. 11.The vaporizer of claim 10, wherein the valve assembly comprises a borewhich sealingly receives an external nozzle of the filling container andincludes at least one valve opening, a seat which receives a valveelement of the filling container which is internal to the nozzle andwhen depressed opens a fluid connection between the nozzle and a body ofthe filling container, and a valve element which is normally biased toclose the at least one valve opening and engaged by the nozzle of thefilling container to displace the valve element and open the at leastone valve opening.
 12. The vaporizer of claim 11, wherein the valveelement includes at least one projection which projects through the atleast one valve opening and is engaged by the nozzle of the fillingcontainer to displace the valve element.
 13. The vaporizer of claim 12,wherein the seat extends proximally of the at least one projection ofthe valve element.
 14. The vaporizer of any of claims 10 to 13, whereinthe filler assembly further comprises a loading/unloading mechanismwhich is operative to engage the filling container and load/unload thefilling container to the valve assembly.
 15. The vaporizer of claim 14,wherein the loading/unloading mechanism comprises an engagement memberwhich is adapted to engage the body of the filling container when thefilling container is inserted into the port of the body of the fillerassembly, and a lever assembly which is coupled to the engagement memberand operative between a first, unlocked position and a second, lockedposition in which the valve assembly is open to allow for filling of thetank by the filling container.
 16. The vaporizer of claim 15, whereinthe lever assembly comprises a lever which is pivotally coupled to thebody of the filler assembly, and a coupling which couples the lever tothe engagement member.
 17. The vaporizer of claim 15 or 16 whendependent upon claim 11, wherein, on moving the lever assembly to thelocked position, the engagement member is moved in relation to the bodyof the filler assembly, which causes the valve element of the fillingcontainer to engage the seat of the valve assembly and the nozzle of thefilling container to engage the valve element of the valve assembly,which causes the valve element of the filling container and the valveelement of the valve assembly to be opened.
 18. The vaporizer of claim17, wherein the nozzle of the filling container engages the valveelement of the valve assembly ahead of the valve element of the fillingcontainer engaging the seat of the valve assembly, such that the valveelement of the filler assembly is opened ahead of the valve element ofthe filling container.
 19. The vaporizer of any of claims 15 to 18,wherein the engagement member includes a slide element which is movablelaterally and acts to engage the body of the filling container to lockthe filling container to the engagement member when the lever assemblyis in the locked position.
 20. The vaporizer of claim 19, wherein theslide element includes a recess or projection, which, when theengagement member is moved to the locked position, is located at adetent to prevent sliding of the slide element.
 21. The vaporizer ofclaim 20, wherein the recess or projection and the detent are configuredsuch that the slide element is moved to an open position when the leverassembly is moved to the unlocked position.
 22. The vaporizer of claim20, wherein the lever assembly is configured to be movable from theunlocked position to a release position by movement of the leverassembly in a sense opposite to that in which the lever assembly ismoved to the locked position, and the recess or projection and thedetent are configured such that the slide element is moved to an openposition when the lever assembly is in the release position.
 23. Thevaporizer of any of claims 19 to 22, wherein the slide element comprisesat least one slide part which is slideably rotated about at least onepivot, and the loading/unloading mechanism further comprises a guidemember which acts to cause the at least one slide part to slidelaterally when the loading/unloading mechanism is moved axially betweenthe unlocked and locked positions.
 24. The vaporizer of claim 23,wherein the slide element comprises first and second slide parts whichare rotated about first and second pivots.
 25. The vaporizer of claim 23or 24, wherein the at least one slide part includes a projection,optionally at a distal end to the pivot, and the guide member includesat least one guide which receives the projection of the at least oneslide part.
 26. The vaporizer of claim 25, wherein the slide elementcomprises first and second slide parts which are rotated about first andsecond pivots, and the guide member includes first and second guideswhich receive the projections of the slide parts.
 27. The vaporizer ofclaim 25 or 26, wherein the at least one guide includes a first guidesection, in which the projection of the at least one slide part islocated when the loading/unloading mechanism is in the unlockedposition, and a second guide section which is disposed laterallyinwardly of the first guide section, in which the projection of the atleast one slide part is located when the loading/unloading mechanism isin the locked position, whereby, when the loading/unloading mechanism isin the unlocked position, the projection of the at least one slide partis located in the first guide section of the at least one guide and theat least one slide part has a first, laterally-outward position whichallows the body of the filling container to be inserted into the bore ofthe valve assembly, and, when the loading/unloading mechanism is movedto the locked position, the projection of the at least one slide part islocated in the second guide section of the at least one guide and the atleast one slide part has a second, laterally-inward position in whichthe body of the filling container is engaged by the at least one slidepart and prevents release of the filling container.
 28. The vaporizer ofany of claims 25 to 27, wherein the at least one guide has the form of atrack, optionally formed by a through slot.
 29. The vaporizer of any ofclaims 10 to 28, wherein the valve assembly comprises a shut-off valvewhich is operable to prevent delivery of the vaporized medium into thegas flow.
 30. The vaporizer of claim 29, wherein the shut-off valveincludes an inlet port which is fluidly connected to the reservoir unitthrough a flow path in the manifold.
 31. The vaporizer of any of claims10 to 30, wherein the valve assembly comprises a metering valve whichregulates an amount of the vaporized medium which is metered into thegas flow in accordance the measured flow rate and/or composition of thegas flow.
 32. The vaporizer of claim 31, wherein the metering valveincludes an outlet port which is fluidly connected to the vaporizedmedium sensing unit through a flow path in the manifold.
 33. Thevaporizer of claim 32 when dependent upon claim 29 or 30, wherein themetering valve includes an inlet port which is fluidly connected to theoutlet port of the shut-off valve through a flow path in the manifold.34. The vaporizer of any of claims 9 to 33, wherein the filler assemblyis thermally connected to the tank of the reservoir unit.
 35. Thevaporizer of any of claims 1 to 34, wherein the gas sensing unitcomprises a gas inlet port through which the flow of gas is delivered,an outlet port from which the flow of gas is delivered, a flow pathwhich fluidly connects the gas inlet port and the outlet port, and atleast one flow rate sensor for sensing the flow rate of the gas flow.36. The vaporizer of claim 35, wherein the gas sensing unit comprises atleast one pair of flow rate sensors.
 37. The vaporizer of claim 36,wherein the gas sensing unit comprises first and second pairs of flowrate sensors.
 38. The vaporizer of any of claims 35 to 37, wherein theat least one flow rate sensor is a mass flow rate sensor, optionally amass flow sensor.
 39. The vaporizer of any of claims 36 to 38, whereinthe flow rate sensors of each pair of flow rate sensors are physicallyseparated along a length of the flow path, providing an upstream sensorand a downstream sensor.
 40. The vaporizer of claim 39, wherein theupstream sensor includes a temperature sensor element and the downstreamsensor includes a heater element and a temperature sensor element, andthe upstream sensor is configured to sense the ambient temperature ofthe gas flow and the downstream sensor is configured to determine anenergy required to maintain the heater element at a required temperaturedifferential in relation to the ambient temperature as determined by theupstream sensor, whereby the flow rate of the gas flow is determinedfrom an energy required to maintain the heater element at the requiredtemperature differential.
 41. The vaporizer of claim 40, wherein thetemperature differential is at least 25° C.
 42. The vaporizer of claim40, wherein the temperature differential is at least 30° C.
 43. Thevaporizer of any of claims 35 to 42, wherein the flow path includes astub path, and the gas sensing unit further comprises at least one gascharacteristic sensor in the stub path for measuring a characteristic ofthe gas flow which is representative of the composition of the gas flow,with the measurement from the gas characteristic sensor being used toprovide a compensation factor for the flow rate of the gas flow asmeasured by the at least one flow rate sensor.
 44. The vaporizer ofclaim 43, wherein the stub path is downstream of the at least one flowrate sensor.
 45. The vaporizer of claim 43, wherein the stub path isupstream of the at least one flow rate sensor.
 46. The vaporizer of anyof claims 43 to 45, wherein the stub path is located in a substantiallylinear section of the flow path.
 47. The vaporizer of any of claims 43to 46, wherein the stub path comprises a sensor cavity at which the atleast one gas characteristic sensor is disposed, and a flow restrictionwhich fluidly connects the flow path to the sensor cavity.
 48. Thevaporizer of claim 47, wherein the at least one gas characteristicsensor is a mass flow rate sensor, optionally a mass flow sensor. 49.The vaporizer of claim 47 or 48, wherein the flow restriction comprisesa flow channel of smaller diameter than the flow path.
 50. The vaporizerof claim 49, wherein the flow restriction has an area of about 1.5 mm²to about 3.5 mm², optionally about 2 mm² to about 3 mm².
 51. Thevaporizer of claim 47 or 48, wherein the flow restriction comprises aporous element, optionally a sintered element, which is disposed in thestub path.
 52. The vaporizer of any of claims 47 to 51, wherein the flowrestriction provides for an average residence time in the sensor cavityof at least 20 seconds, optionally at least 25 seconds, optionally atleast 30 seconds.
 53. The vaporizer of any of claims 47 to 52, whereinthe flow restriction provides for an average residence time in thesensor cavity of at most 60 seconds, optionally at most 40 seconds,optionally at most 35 seconds.
 54. The vaporizer of any of claims 35 to53, wherein the gas sensing unit includes a vaporized medium inlet portin the flow path thereof and downstream of the at least one gascharacteristic sensor, which is fluidly connected to the vaporizedmedium sensing unit.
 55. The vaporizer of any of claims 1 to 54, whereinthe vaporized medium sensing unit comprises a body which includes a flowchannel which includes an inlet port which is fluidly connected to thereservoir unit and an outlet port which is fluidly connected to the gasdelivery unit, and at least one flow sensor for detecting a flow rate ofthe vaporized medium through the flow channel.
 56. The vaporizer ofclaim 55 when dependent upon claim 32, wherein the inlet port of theflow channel is fluidly connected to an outlet port of the meteringvalve by a flow path in the manifold.
 57. The vaporizer of claim 55 or56, wherein the body of the vaporized medium sensing unit comprisesfirst and second body parts, and optionally an annular seal whichsurrounds the flow channel.
 58. The vaporizer of claim 57, wherein atleast one of the first and second body parts of the body of thevaporized medium sensing unit is formed of a material which has athermal conductivity of at least about 100 W/(m.K), optionally at leastabout 150 W/(m.K), optionally at least about 200 W/(m.K), optionallyaluminum.
 59. The vaporizer of any of claims 55 to 58, wherein the bodyof the vaporized medium sensing unit is thermally connected, optionallyfixed directly, to the reservoir unit.
 60. The vaporizer of any ofclaims 55 to 59, wherein the flow channel of the vaporized mediumsensing unit is a linear channel.
 61. The vaporizer of any of claims 55to 60, wherein the flow channel of the vaporized medium sensing unit hasa cross-sectional area of from about 1.5 mm² to about 2.5 mm².
 62. Thevaporizer of any of claims 55 to 61, wherein the at least one flowsensor is a mass flow rate sensor, optionally a mass flow sensor. 63.The vaporizer of any of claims 55 to 62, wherein the vaporized mediumsensing unit includes first and second flow sensors.
 64. The vaporizerof any of claims 1 to 63, wherein the manifold comprises a thermal blockof a material which has a thermal conductivity of at least about 100W/(m.K), optionally at least about 150 W/(m.K), optionally at leastabout 200 W/(m.K), optionally aluminum or an aluminium alloy.
 65. Thevaporizer of claim 64, wherein the manifold comprises a connector whichfluidly connects the vaporized medium sensing unit through a flowpassage in the thermal block to the gas sensing unit.
 66. The vaporizerof claim 65, wherein the connector fluidly connects the outlet port ofthe vaporized medium sensing unit to a vaporized medium inlet port inthe flow path of the gas sensing unit.
 67. The vaporizer of any ofclaims 1 to 66, wherein the volatile medium is a general anaesthetic andthe gas flow is a fresh gas flow, optionally the fresh gas comprises amixture of oxygen, air and nitrous oxide.
 68. A vaporizer for deliveryof a volatile medium to a gas flow, the vaporizer comprising: a gasdelivery unit which receives a flow of gas and provides a flow of gascontaining a metered amount of a vaporized medium; a reservoir unitwhich contains a volatile medium and maintains a supply of the vaporizedmedium, wherein the reservoir unit is selectively fluidly connected tothe gas delivery unit; and a control unit for controlling a flow rate ofthe gas flow and an amount of the vaporized medium which is metered intothe gas flow; wherein the reservoir unit comprises a tank which containsthe volatile medium in liquid form, and a heater which is operative toheat the volatile medium to a vaporization temperature in order tovaporize the volatile medium and maintain an operating pressure in aheadspace of the tank, the tank being formed of a thermally-conductivematerial and providing a thermal mass which is sufficient to maintainthe contained volatile medium at the vaporization temperature for atleast 3 minutes, optionally for at least 5 minutes, followingde-actuation of the heater.
 69. The vaporizer of claim 68, wherein thetank is configured to maintain the contained volatile medium at thevaporization temperature while delivering the vaporized medium at arequired flow rate, optionally a flow rate of at least 1 L/min.
 70. Thevaporizer of claim 68 or 69, wherein the tank is defined in part by amanifold which includes flow paths for the vaporized medium.
 71. Thevaporizer of any of claims 68 to 70, wherein the tank is formed of amaterial which has a thermal conductivity of at least about 100 W/(m.K),optionally at least about 150 W/(m.K), optionally at least about 200W/(m.K), optionally of aluminum or an aluminum alloy.
 72. A vaporizerfor delivery of a volatile medium to a gas flow, the vaporizercomprising: a gas delivery unit which receives a flow of gas andprovides a flow of gas containing a metered amount of a vaporizedmedium; a reservoir unit which contains a volatile medium and maintainsa supply of the vaporized medium, wherein the reservoir unit isselectively fluidly connected to the gas delivery unit; and a controlunit for controlling a flow rate of the gas flow and an amount of thevaporized medium which is metered into the gas flow; wherein thereservoir unit comprises a filler assembly by which volatile medium isintroduced into the tank, the filler assembly comprising a body whichincludes a port for receiving a filling container, a chamber which isfluidly connected to the tank, and a valve assembly which, when opened,is operative to open a fluid connection between the port and the chamberso as to provide for filling of the tank with volatile medium from thefilling container; wherein the filler assembly comprises aloading/unloading mechanism which is operative to engage the fillingcontainer and load/unload the filling container to the valve assembly,the loading/unloading mechanism comprising an engagement member which isadapted to engage the body of the filling container when the fillingcontainer is inserted into the port of the body of the filler assembly,and a lever assembly which is coupled to the engagement member andoperative between a first, unlocked position and a second, lockedposition in which the valve assembly is open to allow for filling of thetank by the filling container.
 73. The vaporizer of claim 72, whereinthe valve assembly comprises a bore which sealingly receives an externalnozzle of the filling container and includes at least one valve opening,a seat which receives a valve element of the filling container which isinternal to the nozzle and when depressed opens a fluid connectionbetween the nozzle and a body of the filling container, and a valveelement which is normally biased to close the at least one valve openingand engaged by the nozzle of the filling container to displace the valveelement and open the at least one valve opening.
 74. The vaporizer ofclaim 73, wherein the valve element includes at least one projectionwhich projects through the at least one valve opening and is engaged bythe nozzle of the filling container to displace the valve element. 75.The vaporizer of claim 74, wherein the seat extends proximally of the atleast one projection of the valve element.
 76. The vaporizer of any ofclaims 72 to 75, wherein the lever assembly comprises a lever which ispivotally coupled to the body of the filler assembly, and a couplingwhich couples the lever to the engagement member.
 77. The vaporizer ofany of claims 72 to 76, wherein, on moving the lever assembly to thelocked position, the engagement member is moved in relation to the bodyof the filler assembly, which causes the valve element of the fillingcontainer to engage the seat of the valve assembly and the nozzle of thefilling container to engage the valve element of the valve assembly,which causes the valve element of the filling container and the valveelement of the valve assembly to be opened.
 78. The vaporizer of claim77, wherein the nozzle of the filling container engages the valveelement of the valve assembly ahead of the valve element of the fillingcontainer engaging the seat of the valve assembly, such that the valveelement of the filler assembly is opened ahead of the valve element ofthe filling container.
 79. The vaporizer of any of claims 72 to 78,wherein the engagement member includes a slide element which is movablelaterally and acts to engage the body of the filling container to lockthe filling container to the engagement member when the lever assemblyis in the locked position.
 80. The vaporizer of claim 79, wherein theslide element includes a recess or projection, which, when theengagement member is moved to the locked position, is located at adetent to prevent sliding of the slide element.
 81. The vaporizer ofclaim 80, wherein the recess or projection and the detent are configuredsuch that the slide element is moved to an open position when the leverassembly is moved to the unlocked position.
 82. The vaporizer of claim80, wherein the lever assembly is configured to be movable from theunlocked position to a release position by movement of the leverassembly in a sense opposite to that in which the lever assembly ismoved to the locked position, and the recess or projection and thedetent are configured such that the slide element is moved to an openposition when the lever assembly is in the release position.
 83. Thevaporizer of any of claims 79 to 82, wherein the slide element comprisesat least one slide part which is slideably rotated about at least onepivot, and the loading/unloading mechanism further comprises a guidemember which acts to cause the at least one slide part to slidelaterally when the loading/unloading mechanism is moved axially betweenthe unlocked and locked positions.
 84. The vaporizer of claim 83,wherein the slide element comprises first and second slide parts whichare rotated about first and second pivots.
 85. The vaporizer of claim 83or 84, wherein the at least one slide part includes a projection,optionally at a distal end to the pivot, and the guide member includesat least one guide which receives the projection of the at least oneslide part.
 86. The vaporizer of claim 85, wherein the slide elementcomprises first and second slide parts which are rotated about first andsecond pivots, and the guide member includes first and second guideswhich receive the projections of the slide parts.
 87. The vaporizer ofclaim 85 or 86, wherein the at least one guide includes a first guidesection, in which the projection of the at least one slide part islocated when the loading/unloading mechanism is in the unlockedposition, and a second guide section which is disposed laterallyinwardly of the first guide section, in which the projection of the atleast one slide part is located when the loading/unloading mechanism isin the locked position, whereby, when the loading/unloading mechanism isin the unlocked position, the projection of the at least one slide partis located in the first guide section of the at least one guide and theat least one slide part has a first, laterally-outward position whichallows the body of the filling container to be inserted into the bore ofthe valve assembly, and, when the loading/unloading mechanism is movedto the locked position, the projection of the at least one slide part islocated in the second guide section of the at least one guide and the atleast one slide part has a second, laterally-inward position in whichthe body of the filling container is engaged by the at least one slidepart and prevents release of the filling container.
 88. The vaporizer ofany of claims 85 to 87, wherein the at least one guide has the form of atrack, optionally formed by a through slot.
 89. A vaporizer for deliveryof a volatile medium to a gas flow, the vaporizer comprising: a gasdelivery unit which receives a flow of gas and provides a flow of gascontaining a metered amount of a vaporized medium; a reservoir unitwhich contains a volatile medium and maintains a supply of the vaporizedmedium, wherein the reservoir unit is selectively fluidly connected tothe gas delivery unit; a gas sensing unit for sensing a flow rate and/orcomposition of the gas flow; and a control unit for controlling a flowrate of the gas flow and an amount of the vaporized medium which ismetered into the gas flow; wherein the gas sensing unit comprises a gasinlet port through which the flow of gas is delivered, an outlet portfrom which the flow of gas is delivered, a flow path which fluidlyconnects the gas inlet port and the outlet port, and at least one pairof low rate sensors for sensing the flow rate of the gas flow; whereinthe flow rate sensors of each pair of flow rate sensors are physicallyseparated along a length of the flow path, providing an upstream sensorand a downstream sensor, and the upstream sensor includes a temperaturesensor element and the downstream sensor includes a heater element and atemperature sensor element, the upstream sensor being configured tosense the ambient temperature of the gas flow and the downstream sensorbeing configured to determine an energy required to maintain the heaterelement at a required temperature differential in relation to theambient temperature as determined by the upstream sensor, whereby theflow rate of the gas flow is determined from an energy required tomaintain the heater element at the required temperature differential.90. The vaporizer of claim 89, wherein the gas sensing unit comprisesfirst and second pairs of flow rate sensors.
 91. The vaporizer of claim89 or 90, wherein the flow rate sensors are mass flow rate sensors,optionally mass flow sensors.
 92. The vaporizer of any of claims 89 to91, wherein the temperature differential is at least 25° C.
 93. Thevaporizer of any of claims 89 to 91, wherein the temperaturedifferential is at least 30° C.
 94. A vaporizer for delivery of avolatile medium to a gas flow, the vaporizer comprising: a gas deliveryunit which receives a flow of gas and provides a flow of gas containinga metered amount of a vaporized medium; a reservoir unit which containsa volatile medium and maintains a supply of the vaporized medium,wherein the reservoir unit is selectively fluidly connected to the gasdelivery unit; a gas sensing unit for sensing a flow rate and/orcomposition of the gas flow; and a control unit for controlling a flowrate of the gas flow and an amount of the vaporized medium which ismetered into the gas flow; wherein the gas sensing unit comprises a gasinlet port through which the flow of gas is delivered, an outlet portfrom which the flow of gas is delivered, a flow path which fluidlyconnects the gas inlet port and the outlet port, and at least one flowrate sensor for sensing the flow rate of the gas flow; wherein the flowpath includes a stub path, and the gas sensing unit further comprises atleast one gas characteristic sensor in the stub path for measuring acharacteristic of the gas flow which is representative of a compositionof the gas flow, with the measurement from the at least one gascharacteristic sensor being used to provide a compensation factor forthe flow rate of the gas flow as measured by the at least one flow ratesensor.
 95. The vaporizer of claim 94, wherein the stub path isdownstream of the at least one flow rate sensor.
 96. The vaporizer ofclaim 94, wherein the stub path is upstream of the at least one flowrate sensor.
 97. The vaporizer of any of claims 94 to 96, wherein thestub path is located in a substantially linear section of the flow path.98. The vaporizer of any of claims 94 to 97, wherein the stub pathcomprises a sensor cavity at which the at least one gas characteristicsensor is disposed, and a flow restriction which fluidly connects theflow path to the sensor cavity.
 99. The vaporizer of claim 98, whereinthe at least one gas characteristic sensor is a mass flow rate sensor,optionally a mass flow sensor.
 100. The vaporizer of claim 98 or 99,wherein the flow restriction comprises a flow channel of smallerdiameter than the flow path.
 101. The vaporizer of claim 100, whereinthe flow restriction has an area of about 1.5 mm² to about 3.5 mm²,optionally about 2 mm² to about 3 mm².
 102. The vaporizer of claim 98 or99, wherein the flow restriction comprises a porous element, optionallya sintered element, which is disposed in the stub path.
 103. Thevaporizer of any of claims 98 to 102, wherein the flow restrictionprovides for an average residence time in the sensor cavity of at least20 seconds, optionally at least 25 seconds, optionally at least 30seconds.
 104. The vaporizer of any of claims 98 to 103, wherein the flowrestriction provides for an average residence time in the sensor cavityof at most 60 seconds, optionally at most 40 seconds, optionally at most35 seconds.
 105. A vaporizer for delivery of a volatile medium to a gasflow, the vaporizer comprising: a gas delivery unit which receives aflow of gas and provides a flow of gas containing a metered amount of avaporized medium; a reservoir unit which contains a volatile medium andmaintains a supply of the vaporized medium, wherein the reservoir unitis selectively fluidly connected to the gas delivery unit; a gas sensingunit for sensing a flow rate and/or composition of the gas flow; avaporized medium sensing unit for sensing a flow rate of the vaporizedmedium; and a control unit for controlling a flow rate of the gas flowand an amount of the vaporized medium which is metered into the gasflow; wherein the vaporized medium sensing unit comprises a body whichis thermally connected to the reservoir unit and includes a flow channelwhich includes an inlet port which is fluidly connected to the reservoirunit and an outlet port which is fluidly connected to the gas deliveryunit, and at least one flow sensor for detecting a flow rate of thevaporized medium through the flow channel.
 106. The vaporizer of claim105, wherein at least part of the body of the vaporized medium sensingunit is formed of a material which has a thermal conductivity of atleast about 100 W/(m.K), optionally at least about 150 W/(m.K),optionally at least about 200 W/(m.K), optionally of aluminum oraluminum alloy.
 107. The vaporizer of claim 105 or 106, wherein the bodyof the vaporized medium sensing unit comprises first and second bodyparts, and optionally an annular seal which surrounds the flow channel.108. The vaporizer of claim 107, wherein at least one of the first andsecond body parts of the body of the vaporized medium sensing unit isformed of a material which has a thermal conductivity of at least about100 W/(m.K), optionally at least about 150 W/(m.K), optionally at leastabout 200 W/(m.K), optionally of aluminum or aluminum alloy.
 109. Thevaporizer of any of claims 105 to 108, wherein the body of the vaporizedmedium sensing unit is fixed directly to the reservoir unit.
 110. Thevaporizer of any of claims 105 to 109, wherein the flow channel of thevaporized medium sensing unit is a linear channel.
 111. The vaporizer ofany of claims 105 to 110, wherein the flow channel of the vaporizedmedium sensing unit has a cross-sectional area of from about 1.5 mm² toabout 2.5 mm².
 112. The vaporizer of any of claims 105 to 111, whereinthe at least one flow sensor is a mass flow rate sensor, optionally amass flow sensor.
 113. The vaporizer of any of claims 105 to 112,wherein the vaporized medium sensing unit includes first and second flowsensors.
 114. The vaporizer of any of claims 105 to 113, furthercomprising: a manifold which includes flow paths for the vaporizedmedium and fluidly connects the reservoir unit and the vaporized mediumsensing unit.
 115. The vaporizer of claim 114, wherein the manifoldcomprises a thermal block, optionally of a material which has a thermalconductivity of at least about 100 W/(m.K), optionally at least about150 W/(m.K), optionally at least about 200 W/(m.K), optionally ofaluminum or aluminum alloy.
 116. The vaporizer of claim 115, wherein themanifold comprises a connector which fluidly connects the vaporizedmedium sensing unit through a flow passage in the thermal block to thegas sensing unit.
 117. The vaporizer of claim 116, wherein the connectorfluidly connects the outlet port of the vaporized medium sensing unit toa vaporized medium inlet port in the flow path of the gas sensing unit.118. The vaporizer of any of claims 114 to 117, wherein the reservoirunit comprises a tank which contains the volatile medium in liquid form,and a heater which is operative to heat the volatile medium to avaporization temperature in order to vaporize the volatile medium andmaintain an operating pressure in a headspace of the tank.
 119. Thevaporizer of claim 118, wherein the tank is formed of athermally-conductive material and, optionally together with themanifold, provides a thermal mass which is sufficient to maintain thecontained volatile medium at the vaporization temperature for at least 3minutes, optionally for at least 5 minutes, following de-actuation ofthe heater.
 120. The vaporizer of claim 119, wherein the tank isconfigured to maintain the contained volatile medium at the vaporizationtemperature while delivering the vaporized medium at a required flowrate, optionally at a flow rate of least 1 L/min.
 121. The vaporizer ofany of claims 118 to 120, wherein the tank is defined in part by themanifold.